docs: add details about creating new VMs

docs/maintainers.md

@@ -92,7 +92,9 @@ If this is your first deploy, make sure another maintainer is around to assist.
 
 The bundled documents are available at downloads.devdocs.io and the documents themselves at documents.devdocs.io.  Download and document requests are proxied to S3 buckets devdocs-downloads.s3.amazonaws.com and devdocs-documents.s3.amazonaws.com respectively.
 
-If there's ever a need to create a new proxy VM (and the `devdocs-proxy` snapshot is not available) then the new vm should be provisioned as follows:
+New proxy VMs should be created from the `devdocs-proxy` snapshot.  Before adding them to the load-balancer, it's necessary to add their IP addresses to the aws:SourceIp lists for both buckets, or their requests will be rejected.
+
+When creating a new proxy VM and the `devdocs-proxy` snapshot is not available, then the new vm should be provisioned as follows:
 
 ```bash
 # we need at least nginx 1.19.x